At present, the energy crisis and global warming have become major issues affecting human survival and development, and thus the demand for development of clean renewable energy such as wind energy and solar energy is increasing. However, due to the instability of power stations of wind and solar power generation and centralized supplying solution therefrom, one precondition for large-scale use of these energy sources is the development of suitable high-capacity high-power energy storage devices. Using the transportation system with the most representative energy storage needs as an example, the most successful solution at present is a secondary chemical battery (such as a lithium-ion battery) used in small electric vehicles. Such a battery has a high specific energy but a low power density of no more than 500 W/kg, and generates internal heat and warms up during a discharge under a large pulse current or a fast charging, thus reducing the service life of the battery, and even causing danger. For buses, freight cars, and high speed railways with higher power needs, it is clear that secondary chemical batteries are unable to meet such needs. This requires a new energy storage system which can provide both a high capacity and a high power.
An electrochemical capacitor, also known as a supercapacitor, is a new energy storage device developed since the 1970s and 1980s, and a new energy storage system with a performance and a working mechanism between electronic capacitors and chemical batteries. It dramatically increases the upper limit of the capacitance by 3 to 4 orders of magnitude, reaching a high capacitance of a Farad (F) level. Currently, commonly used secondary batteries such as nickel cadmium batteries, nickel metal hydride batteries, and lithium ion batteries have a high specific energy but a low power density of no more than 500 W/kg, and generate internal heat and warm up in a large pulse current discharge or a fast charging, thus reducing the service life of such batteries, and even causing danger. Fuel cells also have defects of low power density, and low resistance to high pulse charging and discharging. The supercapacitor, as a new energy storage element, has a performance between conventional electrostatic capacitors and batteries, with the following features: 1. high energy density (1˜10 Wh/kg); 2. high power density (2 kw/kg), and high-current discharge (several thousand amperes); 3. long service life (more than 100,000 times); 4. wide temperature range (−40˜70° C.); 5. high charging speed (tens of seconds); 6. long shelf life (several years); 7. maintenance-free and environmentally friendly. These features meet the demands for new energy and energy storage devices with the higher energy density and power density required by the development of science and technology and improvement of human living standards, and have important and broad application prospects in the fields of mobile communications, information technology, consumer electronics, electric vehicles, and aerospace, and are thus receiving more and more attention in the world.
There are two types of supercapacitor materials based on the working mechanisms thereof. One type is a double-layer supercapacitor material, which stores energy mainly based on a directed migration of electrolyte ions in a surface of the electrode. Therefore, the specific surface area of the electrode material has an important influence on the double layer capacitance. Carbon materials (e.g. carbon nanotubes, activated carbon, graphene, etc.) are widely used as electrode materials for double-layer capacitors due to their large specific surface area and stability. Activated carbon, a representative of carbon materials, is now used in the vast majority of the commercial supercapacitors. Due to limitations of the double-layer energy storage, this type of capacitor generally has a specific capacitance of less than 300 F/g, making the energy density thereof much less than the application requirements. Another type, called redox Faraday pseudocapacitive material, stores energy mainly by means of the redox reaction of the electrode material. The energy storage mechanism of rapid insertion and extraction of ions in the surface of the material makes such material have a higher specific capacitance. Representatives of the material are certain transition metal oxides (such as ruthenium oxide, manganese dioxide, nickel oxide, cobaltosic oxide, etc.) and conductive polymers (such as polyaniline, polypyrrole, etc.). However, the commonly used supercapacitor materials above all have problems in practical application, for example, double-layer capacitor materials such as carbon material have shortcomings of low capacitance and high production cost; since their electrochemical window can only be positive, manganese dioxide and nickel oxide cannot be made into a symmetric device, have a large internal resistance which restricts the power density, and greatly attenuate after thousands of cycles (nickel oxide attenuates in capacitance by 60% after 1,000 cycles), thus greatly limiting the application on devices; ruthenium oxide has a high specific capacitance and a symmetric electrochemical window but has a high cost because the noble metal ruthenium is expensive; and conductive polymers have a large internal resistance which restricts the power density of device, thus being limited in practical applications.
Unmodified white titanium dioxide material, as a variable valence oxide of lightest molar mass, can undergo a Ti4+/Ti3+ redox pseudocapacitive reaction theoretically and has a theoretical capacitance up to 2,000 F/g, but actually exhibits a very low specific capacitance (<0.1 mF cm−2), and a poor electrical conductivity, and has never been considered as an available active material for supercapacitors. At present, titanium dioxide is used in supercapacitors substantially in such a manner that titanium dioxide is nano-structured, and compounded with conventional supercapacitor materials (such as manganese oxide, polymers, etc.), wherein titanium dioxide merely serves as a supporter with a large specific surface area to improve the performance of the conventional capacitors. Yat Li has reported that MnO2 is supported on hydrogenated black titanium dioxide nanowires to give an asymmetric supercapacitor device (Lu, X.; Yu, M.; Wang, G.; Zhai T.; Xie, S.; Ling, Y.; Tong, Y.; Li, Y. Adv. Mater., 2013, 25, 267-272), however, the titanium dioxide nanowires are used to improve the conductivity of the current collector by hydrogenation, rather than serving as an active materials. Even the surface conductivity and density of charge carriers are improved after a surface hydrogenation treatment, the electrode with titanium dioxide as the main active material only has a capacitance of ˜1-3.2 mF cm−2 according to a report (Lu, X.; Wang, G.; Zhai T.; Yu, M.; Gan, J.; Tong, Y.; Li, Y. Nano Lett, 2012, 12, 1690-1696), which is much less than that of conventional supercapacitor materials. Recently, Yadong Li has reported a nitrided titanium dioxide supercapacitor. However, the capacitance thereof only improves in the case that titanium dioxide is nitrided at a high temperature to generate titanium nitride, while a sample without a titanium dioxide component due to a low temperature nitriding treatment does not have an improved capacitance. It is thus clear that the active substance is titanium nitride rather than titanium dioxide (Moon, G.; Joo, J.; Dahl, M; Jung, H.; Yin, Y. Adv. Funct. Mater., 2013, DOI:10.1002/adfm. 201301718). So far, a better capacitance performance has only been obtained in the case that high-temperature hydrogenated titanium oxide arrays serve as a current collector and active materials manganese oxide are supported thereon (Lu, X.; Yu, M.; Wang, G.; Zhai T.; Xie, S.; Ling, Y.; Tong, Y.; Li, Y. Adv. Mater., 2013, 25, 267-272).